1. Field of the Invention
This invention relates in general to thermal spray guns used for thermal spraying a substrate with a coating applied in a high velocity flow stream.
2. Description of the Prior Art
Thermal spray guns are used in processes for thermal spraying substrates with coatings transported in high energy flow streams. Thermal spraying has also been known as flame spraying, metalization, high velocity oxy-fuel thermal spraying (H.V.O.F.), and high velocity air-fuel thermal spraying (H.V.A.F.). Coating materials are typically metals, ceramics, or cermet types of materials. The high energy flow streams typically include a carrier gas for propelling and transporting the coating material to a substrate target at high velocities. The coating material may be transported at supersonic velocities, often several times the speed of sound. In fact, some thermal spray guns and thermal spray processes determine proper operation of the gun by counting the number of shock diamonds appearing in the gas jet formed by the high energy flow stream exiting the gun.
Coatings applied by thermal spraying are thought to adhere to a substrate primarily by mechanical adhesion resulting from coating particles colliding with the surface of a substrate at high velocities. It is also theorized that bombarding a substrate with high velocity coating particles results in some of the kinetic energy of the coating particles being converted to heat when the coating particles impact with the substrate. This heat from converted kinetic energy is believed to aid in bonding the coating material to the substrate.
A thermal spray carrier gas is typically provided by a high velocity flame-jet resulting from combustion of a fuel which releases heat and generates a high temperature pressurized gas, which is the carrier gas. Thermal spray guns typically utilize combustion components, or reactants, such as oxygen and propane, oxygen and hydrogen, oxygen and kerosene, and kerosene and air. A fuel and an oxygen source are injected into a combustion chamber where they react in a combustion reaction under pressure and temperature to generate the high temperature pressurized gas, which is directed from the combustion chamber and into a high velocity flow stream. Coating materials, such as metals, ceramics, or cermets, are inserted into the flow stream. The high temperature pressurized gas is directed from the combustion chamber and down a flow nozzle to propel the coating material particles into a targeted substrate. Often, several shock diamonds appear in the high velocity flow stream exiting the thermal spray gun to indicate that the high temperature pressurized gas is travelling towards the targeted substrate at several times the speed of sound.
An example of a thermal spray gun is disclosed in U.S. Pat. No. 4,343,605, invented by James A. Browning, and issued Aug. 10, 1982. Additionally, several other Browning patents disclose further advances in thermal spray guns, such as:
U.S. Pat. No. 4,370,538, issued Jan. 25, 1983;
U.S. Pat. No. 4,416,421, issued Nov. 22, 1983;
U.S. Pat. No. 4,540,121, issued Sep. 10, 1985;
U.S. Pat. No. 4,568,019, issued Feb. 4, 1986;
U.S. Pat. No. 4,593,856, issued Jun. 10, 1986;
U.S. Pat. No. 4,604,306, issued Aug. 5, 1986;
U.S. Pat. No. 4,634,611, issued Jan. 6, 1987;
U.S. Pat. No. 4,762,977, issued Aug. 9, 1988;
U.S. Pat. No. 4,788,402, issued Nov. 29, 1988;
U.S. Pat. No. 4,836,447, issued Jun. 6, 1989; and
U.S. Pat. No. 4,960,458, issued Oct. 2, 1990.
The above referred U.S. Patents, including U.S. Pat. No. 4,343,605 and U.S. patent application Ser. No. 08/007,264, filed Jan. 2, 1993, are hereby incorporated by reference as if fully set forth herein.
An example of a Browning thermal spray gun is the Browning H.V.A.F. Model 250 Thermal Spray Gun, or the smaller Browning H.V.A.F. Model 150 Thermal Spray Gun. These thermal spray guns pass combustion air about the exterior of a flow nozzle to both cool the flow nozzle, and preheat the combustion air. Preheating the combustion air by passing it along the flow nozzle and within a combustion chamber housing prevents some of the heat loss experienced in some prior art thermal spray gun having liquid cooling systems. However, preheating combustion air by passing it along the flow nozzle cools the flow nozzle to temperatures well below the high energy flow stream, which results in drawing off excessive thermal energy from the high energy flow stream. Often, prior art thermal spray guns carry off heat from flow nozzles by cooling with either a coolant liquid, forced air, or ambient air passing about the nozzle by convection, all of which carry off heat transferred to the flow nozzle from the flow stream. Excessive cooling results in reduced deposit efficiencies.
Testing with the Browning Model 250 yielded a coating deposit efficiency of approximately 20% when using a Union Carbide Number 489-1 coating material of 88% tungsten carbide with a 12% cobalt matrix, which has a particle size between 10 to 45 microns and the 12% cobalt added as a binder. A 20% coating deposit efficiency means that of 10 pounds of coating material applied to a targeted substrate, only 2 pounds were found to adhere to the substrate.
Although most thermal spray guns include some fine tuning capabilities for controlling the thermal spray process by adjusting the fuel and combustion air flow rate into the thermal spray gun, still only a narrow band width of particle sizes can be effectively sprayed with these thermal spray guns. For example, tests have shown that the Browning Model 250 and Model 150 can only be effectively utilized to apply coating materials having particle sizes of in the range between 10 to 45 microns. When particles approach sizes larger than 45 microns, the deposit efficiency is reduced even lower than 20% when using kerosene as a fuel. It should be noted that if larger particle sizes could be used, particles propelled towards a target at a specific velocity would have an additional amount of kinetic energy over that of a smaller particle size, resulting in conversion of the additional kinetic energy into additional thermal heat upon impact with the targeted substrate.
Further, testing showed that the Browning Model 150 thermal spray gun, which is a small diameter thermal spray gun, could not be operated for extended periods of time to thermal spray a substrate with a coating. The inner sleeve which lines the combustion chamber would deteriorate when exposed to the high temperatures at which the Browning Model 150 was operated. The air flowing through the combustion chamber housing annular space between the combustion chamber outer sleeve and the combustion chamber inner sleeve did not adequately cool the combustion chamber inner sleeve when the smaller combustion chamber of the Model 150 was raised to operating temperatures. Improvements are desired to increase the burn efficiency for smaller thermal spray guns, such as the Browning Model 150.
Typically, liquid fuels such as kerosine and diesel do not burn cleanly when they are used to operate prior art thermal spray guns. Usually, these liquid fuels will leave a buildup of combustion deposits on the interior of a thermal spray gun. Buildup of combustion deposits have led to such problems as excessive heat building up within liners of combustion chambers, causing catastrophic failure of combustion chamber components. If thermal spray guns could be operated on liquid fuels without a buildup of combustion deposits on the thermal spray gun interior, this would extend the service life of thermal spray guns. Thus, in addition to cost savings which could be achieved by burning liquid fuels rather than more expensive gaseous fuels, thermal spray guns would be more economical to operate if the service life of combustion chamber components could be extended.